Gas-phase olefin polymerization process

ABSTRACT

A continuous process for the olefin polymerization in a fluidized bed reactor, said process comprising continuously passing a gaseous stream comprising one or more α-olefin monomers through the fluidized bed in the presence of a polymerization catalyst under reactive conditions, withdrawing polymeric product and unreacted fluids from the reactor, cooling part of said unreacted fluids below the dew point to form a two-phase mixture of gas and condensed liquid and reintroducing said two-phase mixture into the reactor, the process being characterized in that said twophase mixture is reintroduced under the distribution plate of the reactor so that a part of condensed liquid is separated from the gas and is successively fed above the fluidized bed through an external pipe connecting the bottom of the reactor to a point situated above the upper limit of the fluidized bed of polymer particles.

The present invention relates to a process for the gas-phasepolymerization of olefins in a fluidized bed reactor.

The development of catalysts with high activity and selectivity of theZiegler-Natta type and, more recently, of the metallocene type has ledto the widespread use on an industrial scale of processes in which theolefin polymerization is carried out in a gaseous medium in the presenceof a solid catalyst. An example of said gas-phase polymerizationprocesses involves the use of a fluidized bed reactor wherein a bed ofpolymer particles are maintained in a fluidized state by the upward flowof gaseous monomer. During the polymerization, fresh polymer isgenerated by catalytic polymerization of the monomer and polymer productis continuously withdrawn to maintain the bed at a constant volume.Industrial processes employ a distribution plate to dispense thefluidizing gas to the bed, and to act as a support for the bed when thesupply of gas is cut off. The polymer product is generally withdrawnfrom the reactor via a discharge conduit arranged in the lower portionof the reactor near the distribution plate. The fluidized bed comprisesa bed of growing polymer particles and catalyst particles. This reactionmixture is maintained in a fluidized condition by the continuos upwardflow of a fluidizing gas which comprises recycle gas and monomermake-up. The fluidizing gas enters the bottom of the reactor and ispassed through the distribution plate to the fluidized bed.

The polymerization of olefins is an exothermic reaction and it istherefore necessary to provide means to cool the bed to remove the heatof polymerization. In the absence of such cooling the bed would increasein temperature until, for example, the catalyst turns inactive or thepolymer particles are partially fused. In a fluidized bedpolymerization, the preferred method for removing the heat ofpolymerization is by feeding to the polymerization reactor a recycle gasstream at a temperature lower than the desired polymerizationtemperature. Such a gas stream, while passing through the fluidized bed,allows conducting away the heat of polymerization. The recycle gasstream is withdrawn from the upper zone of the reactor, cooled bypassage through an external heat exchanger and then recycled to thereactor. The temperature of the recycle gas can be adjusted in the heatexchanger to maintain the fluidized bed at the desired polymerizationtemperature. According to this method of reactor cooling, the recyclegas stream generally comprises, besides the gaseous monomers, also inertand diluent gases, such as propane, and gaseous chain transfer agents,such as hydrogen. Thus, the recycle gas stream serves to supply themonomer to the bed, to fluidize the bed and also to maintain the bed atthe desired temperature. Monomers consumed by the polymerizationreaction are normally replaced by adding make-up gas to the recycle gasstream.

It is well known that the space time yield, in terms of weight ofpolymer produced per unit volume of reactor and per unit time, in acommercial gas fluidized bed reactor is limited by the maximum rate atwhich the heat of polymerization can be removed from the reactor. Therate of heat removal can be increased, for example, by increasing thevelocity of the recycle gas and/or reducing the temperature of therecycle gas. However, there is a limit to the velocity of the recyclegas that can be employed in the industrial practice. Beyond this limitthe bed can become unstable or even lift out of the reactor along withthe gas stream, leading to blockage of the recycle line and damage tothe recycle gas compressor. There is also a limit on the extent to whichthe recycle gas can be cooled in practice. This is primarily determinedby economic considerations, and in practice is normally determined bythe temperature of the industrial cooling water available on site.Refrigeration can be employed if desired, but this adds to theproduction costs.

Thus, in the commercial practice the use of cooled recycle gas as theonly means of removing the heat of polymerization from the gas fluidizedbed reactor has the disadvantage of limiting the maximum productionrate. In order to overcome this disadvantage, different methods havebeen suggested for removing the heat of polymerization from a fluidizedbed polymerization process.

EP 89 691 relates to a process for increasing the space time yield in acontinuous gas fluidized bed process for the polymerization of olefins.According to this patent, the recycle gas stream is intentionally cooledto a temperature below the dew point of the recycle gas stream toproduce a two-phase gas/liquid mixture under conditions such that theliquid phase of said mixture will remain entrained in the gas phase ofsaid mixture. The heat of polymerization is removed by introducing saidtwo-phase mixture into the reactor at a point in the lower region of thereactor, and most preferably at the bottom of the reactor to ensureuniformity of the fluid stream passing upwardly through the fluidizedbed. The evaporation of the liquid phase takes place inside thepolymerization bed and this ensures a more effective removal of the heatof polymerization. This technique is referred to as operation in the“condensing mode”. By operating in the “condensing mode”, the coolingcapacity of the recycle stream is increased by both the vaporization ofthe condensed liquids entrained in the recycle stream and as a result ofthe greater temperature gradient between the entering recycle stream andthe reactor. The specification of EP 89 691 states that the quantity ofcondensed liquid contained in the gas phase should not exceed about 20%by weight and preferably should not exceed about 10% by weight, providedthat the velocity of the two-phase recycle stream is high enough to keepthe liquid phase in suspension in the gas and to support the fluidizedbed in the reactor. The recycle system described in this patent is suchthat all the condensed liquid is introduced in the lower region of thefluidized bed. As a consequence, the cooling capacity of the recyclestream in the upper region of the fluidized bed is very poor.

U.S. Pat. No. 4,588,790 also relates to a polymerization process ofolefins in a fluidized bed reactor by operating in the “condensingmode”. This patent deals with the problem of the carryover of solidparticles in the recycle stream. The smallest polymer particles, alsocalled the “fines”, are carried over by the gaseous stream and arerecycled to the reactor together with the liquid phase so that they cancreate undesiderable “mud” inside the fluidized bed. This “mud” can beformed by the wetting of the fines, their agglomeration and accumulationas “chunks” in regions of relatively low velocity in the system, forinstance near the gas distribution plate. In order to minimize theformation of “mud”, the specification of U.S. Pat. No. 4,588,790 statesthat the weight ratio of liquid to solids in the recycle stream shouldnot be less than about 2:1. The higher is said ratio, the lower is theprobability of formation of chunks when the “condensing mode” isadopted. As regards the introduction of the condensed liquid inside thefluidized bed, this patent discloses the possibility of splitting thetwo-phase mixture in more separated streams, and some of them can beintroduced directly into the fluidized bed. However, the gasesintroduced below the fluidized bed must be sufficient to support thefluidized bed and to maintain it in a fluidization condition.Accordingly, the major portion of the two-phase gas/liquid mixture mustnecessarily be fed at a point below the fluidized bed: this limitationmakes ineffective the cooling capacity of the recycle stream in theupper region of the fluidized bed.

EP 699 213 relates to a continues fluidized bed process for thepolymerization of olefins operating in the condensing mode. According tothis patent, after the cooling of the recycle stream at a temperaturebelow its dew point, at least part of the condensed liquid is separatedby the gas phase and introduced directly into the fluidized bed. Inorder to gain the maximum benefit in term of cooling of the fluidizedbed, the separated liquid must be introduced in the region of the bedthat has substantially reached the temperature of the gaseous streamleaving the reactor. The introduction of the separated liquid may becarried out at a plurality of points within this region of the fluidizedbed, and these points may be at different heights within this region.For example, the points of introduction of the liquid into the fluidizedbed may be approximately 50-70 cm above the fluidization grid. Injectionmeans are required, preferably nozzles, arranged such that they protrudesubstantially vertically into the fluidized bed or may be arranged suchthat they protrude from the walls of the reactor in a substantiallyhorizontal direction. The presence of said injection means could causeundesirable turbulence and serious risk of fouling due to the creationof dead spots in the vicinity of the nozzles or similar injection means.Another drawback of this process is due to the fact that additionalequipment is required in the recycle line for separating the condensedliquid from the gas phase, in particular cyclone separators, demistertype gas-liquid separators or liquid scrubbers. Furthermore, a pump mustbe provided downstream the separator in order to allow the injection ofthe separated liquid along the axis of fluidized bed. As a consequence,the recycle system described in this patent increases the plant costsand the complexity of the plant setup.

Also the process of U.S. Pat. No. 6,306,981 requires a separation stepwherein at least part of the condensed liquid is separated from the gasphase by means of a separator. According to the teaching of this patent,the separated liquid is transferred by a pump to the reactor andintroduced peripherally in the upper portion of the fluidized bed at alocation in proximity of the reactor walls. A film of liquid is formedwhich flows downward along the reactor walls. The vaporization of saidliquid film cools the upper region of the fluidized bed without causingan undesirable turbulence in the central core region of the fluidizedbed. The process described in this patent improves the level of coolingof the fluidized bed, however also in this case both the gas/liquidseparator and the pump along the recycle line increase the plant costsand the complexity of the plant setup.

According to EP 825 204, the gas/liquid mixture obtained by cooling therecycle stream is transferred to the bottom of a fluidized bed reactor,where the condensed liquid is separated from the gaseous stream in aseparator which is integral with the fluidized bed reactor. The liquidis withdrawn from the bottom of said integral separator and isintroduced into the lower part of the fluidized bed. Also this processrequires the use of injection means, preferably nozzles, arranged so asto protrude substantially vertically into the fluidized bed or toprotrude from the walls of the reactor in a substantially horizontaldirection. The presence of said injection means can cause undesirableturbulence and serious risk of fouling due to the creation of dead spotsin the vicinity of the nozzles or other injection means. Furthermore,also this process requires the presence of a pump, downstream theintegral separator, for introducing the condensed liquid inside thefluidized bed and for maintaining a continuous circulation and stirringof liquid at the bottom of the integral separator.

It would be desirable to improve the process described in EP 825 204 byavoiding the use of liquid injection means directly protruding into thefluidized bed of polymer particles and, at the same time, simplifyingthe equipment involved in the recycle line. It has now been found that aparticular arrangement in the recycle line of the gas/liquid mixtureallows to obtain a more effective cooling of a fluidized bed reactorwith the advantages of reducing the complexity of the plant setup andavoiding the use of injection means directly protruding into thefluidized bed.

It is an object of the present invention a continuous process for theolefin polymerization in a fluidized bed reactor, said processcomprising continuously passing a gaseous stream comprising one or moreα-olefin monomers through the fluidized bed in the presence of apolymerization catalyst under reactive conditions, withdrawing polymericproduct and unreacted fluids from the reactor, cooling part of saidunreacted fluids below the dew point to form a two-phase mixture of gasand condensed liquid and reintroducing said two-phase mixture into thereactor, the process being characterized in that:

said two-phase mixture is reintroduced under the distribution plate ofthe reactor so that a part of condensed liquid is separated from the gasand is successively fed above the fluidized bed through an external pipeconnecting the bottom of the reactor to a point situated above the upperlimit of the fluidized bed of polymer particles.

In the present invention the term “external pipe” is referred to a piperunning outside the fluidized bed reactor, the inlet of said pipe beingplaced at the bottom end of the reactor, the outlet of said pipe beingplaced above the fluidized bed of polymer particles.

According to an embodiment of the present invention, the two-phasemixture formed by cooling the unreacted fluids at a temperature belowthe dew point is reintroduced into the fluidized bed reactor along adirection which is tangential to the reactor walls. Due to thistangential inlet, a part of condensed liquid is separated from the gasby a “centrifugal effect” involved in the zone underlying thedistribution plate. Generally, the inlet point of the two-phase mixtureinto the reactor is situated close to and just below the distributionplate in order to exploit all the space underlying the distributionplate to carry out the above separation.

According to another embodiment, the above separation is achieved bymeans of one or more baffles placed near the point of reintroduction ofthe two-phase mixture into the reactor. In this case, a part ofcondensed liquid is separated from the gas by coalescence of liquiddroplets on said baffles and consequent fall by gravity.

In both the embodiments, the separated liquid collects at the bottom ofthe zone underlying the distribution plate before entering the externalpipe. The amount of liquid entering the external pipe is generallycomprised in the range of from 20 to 50% by weight of the totalcondensed liquid. On the other hand, the remaining part of condensedliquid, generally comprised between 50% and 80% by weight of the totalcondensed liquid, enters the fluidized bed passing through the slots ofthe distribution plate.

Only a partial and raw separation of the liquid from the gas is carriedout in the reactor zone underlying the distribution plate, so that atwo-phase mixture enriched in liquid collects in proximity of the inletof the external pipe and runs through said pipe, while a two-phasemixture enriched in gas passes through the distribution plate. Thelatter provides the fluidizing gas needed to keep the polymer bed in afluidization state.

Many advantages can be accomplished by carrying out the process of theinvention. In the first place, the introduction of condensed liquidabove the fluidized bed improves the cooling of the upper region of thefluidized bed without causing any turbulence and interference with thefluidization conditions of the polymer bed. Simultaneously, theremaining part of condensed liquid moves upwards through thedistribution plate so as to effect a good cooling of the lower region ofthe bed. In order to obtain these advantages, it is essential to arrangea pipe connecting the bottom of the reactor to a region of the reactorsituated over the fluidized bed.

According to the present invention, the liquid flows upward in theexternal pipe without requiring pumping devices. In fact, the pressuregradient Δp existing between the zone underlying the distribution plateand the zone overlying the polymer fluidized bed causes freely the fluidto flow upwards along said pipe. In a manner unknown in the prior artembodiments said pressure gradient, which is made available by therecycle compressor, can be exploited to introduce the condensed liquidinto the reactor without using additional pumps or similar devices.

As known, a fluidized bed reactor includes at its top a velocityreduction zone, which is generally of increased diameter compared to thediameter of the fluidized bed portion of the reactor. At the outlet ofthe external pipe, the liquid is preferably introduced into thefluidized bed reactor at a point situated above the upper limit of thefluidized bed and below the velocity reduction zone. The liquid can besimply poured onto the top of the fluidized bed or can be sprayed ontothe top of the fluidized bed by means of injection devices, such as aplurality of nozzles. One or more feeding points placed along acircumference overlying the fluidized bed can be provided.

It is preferred to operate in such a way that an annular flow of liquidis established inside the external pipe, while the central sectionthereof is preferably occupied by the upward flow of gas. By sooperating, the liquid strictly adheres to the walls of the pipe so as toreduce the probability of clogging the pipe. In order to form saidliquid annular film, the diameter of the external pipe should besuitably selected taking into account the liquid flow rate and thepressure gradient Δp existing between the inlet and the outlet of theexternal pipe. It has been found that the formation of said liquidannular flow is favoured when the liquid entering the external pipe isin an amount comprised from 10 to 20% by weight with respect to theamount of gas entering said pipe. As regards the diameter of theexternal pipe, this parameter is generally selected at a value of lessthan 0.15 D_(R), where D_(R) is the diameter of the fluidized bedreactor. Above this upper limit, an excessive amount of gas enters theexternal pipe and, as a consequence, the gas passing through thedistribution plate is not enough to support the fluidized bed ofpolymer. A suitable range for the diameter of the external pipe is from0.01 to 0.15 D_(R), preferably from 0.02 to 0.08 D_(R).

A further advantage of the invention is that the centrifugal effectinvolved by the tangential inlet of the recycle stream favours aconcentration of the recycled “fines” at the bottom part of the reactor,so that most of the fines are forced to run the external pipe. As aconsequence, the process of the invention allows to by-pass the “fines”to the upper region of the fluidized bed reactor, thereby minimizing theamount of fines discharged via the product discharge valve located inthe bottom region of the fluidized bed reactor. By doing so, thecatalyst yield can be increased.

According to the invention, the gaseous stream which is continuouslypassed through the fluidized bed comprises one or more α-olefinmonomers. Suitable α-olefin monomers are those of formula CH₂═CHR, whereR is hydrogen or a hydrocarbon radical having 1-12 carbon atoms. Saidgaseous stream can also include one or more alkanes or cycloalkanes asinert condensable gases. Preferably C₄-C₈ alkanes or cycloalkanes areused as inert condensable gases, in particular butane, pentane orhexane.

Generally, the recycle stream is cooled at a temperature under the dewpoint to a such extent that it is formed an amount of condensed liquidnot exceeding 20% by weight of the total amount of liquid and gas.Preferably, the amount of condensed liquid does not exceed 12% by weightof the total amount of liquid and gas. The condensed liquid comes fromthe condensable monomers, e.g. propylene, butene-1, hexene-1, octene andthe inert condensable gases, e.g. propane, butane, pentane or hexane.

The present invention is now described in detail with reference to theattached FIGS. 1-2, which are given for illustrative purpose notlimiting the scope of the invention.

FIG. 1 shows a fluidized bed reactor comprising a reactor body 1including a fluidized bed 2 of polymer, a fluidization plate 3 and avelocity reduction zone 4. The velocity reduction zone 4 is generally ofincreased diameter compared to the diameter of the fluidized bed portionof the reactor. The gaseous stream leaving the top of the velocityreduction zone 4 comprises, besides the unreacted monomers, also inertcondensable gases, such as isopentane, as well as inert non-condensablegases, such as nitrogen. Said gaseous stream is compressed, cooled andrecycled to the bottom of the fluidized bed reactor: from the top of thevelocity reduction zone 4 the gaseous stream is transferred via recycleline 5 to a compressor 7 and then to a heat exchanger 8. If appropriate,the recycle line 5 is equipped with a line 6 for feeding monomers,molecular weight regulators and, optionally inert gases. Passing throughthe heat exchanger 8, the gaseous stream is cooled below its dew pointto form a two-phase mixture of gas and condensed liquid. Said two-phasemixture obtained at the outlet of the heat exchanger 8 is transferred tothe bottom of the fluidized bed reactor via line 9. The inlet point ofline 9 in the reactor is situated just below the distribution plate 3and the direction of the inlet of said line 9 is tangential to thereactor wall. Said tangential inlet favours a “centrifugal effect” inthe zone underlying the distribution plate 3 so that part of the liquidcontained in the two-phase mixture is collected at the bottom part ofsaid zone. As a consequence, a liquid/gas mixture enriched in liquidflows through the external pipe 10, while a liquid/gas mixture enrichedin gas passes through the slots of the distribution plate 3. In this wayan amount of upwardly flowing gas sufficient to maintain the bed in afluidized condition is ensured.

The inlet of the external pipe 10 is placed at the bottom end of thefluidized bed reactor while the outlet of the pipe 10 is situated abovethe upper limit of the fluidized bed 2 and below the velocity reductionzone 4. No pumping devices are required to guarantee the flow ofcondensed liquid upwardly into the external pipe 10. At the outlet ofthe external pipe 10, the liquid is sprayed onto the top of thefluidized bed 2 by means of injection devices (not shown).

Generally, the various catalyst components are fed to the reactorthrough a line 11 that is preferably placed in the lower part of thefluidized bed 2. The polymer can be discharged through a line 12 placedat the bottom of the fluidized bed 2. Make-up monomers can be alsointroduced into the reactor in either liquid or gaseous form via line13.

FIG. 2 is a sectional view of the reactor body 1 at a level situatedjust below the distribution plate 3 in correspondence of the inlet pointof line 9 in the reactor: as shown, the direction of the inlet of line 9is tangential to the reactor wall.

The process of the invention is operated with a gas velocity in thefluidised bed which must be greater or equal to that required for thefluidisation of the bed. The polymerisation is preferably carried out byusing a gas velocity in the range 40 to 100 cm/sec, more preferably 50to 80 cm/sec. The distribution plate 3 can be of conventional design,for instance, a flat or dished plate perforated by a plurality of slotsdistributed more or less uniformly across its surface. Slots ofrectangular shape and having a large opening, for instance 12×40 mm, arepreferably adopted in the present invention: these slots foster thepassage of a gas flow containing droplets of entrained liquid.

As it can be easily understood from the embodiment shown in FIG. 1, theremarkable advantages of the process of the invention in term of reactorcooling are obtained simplifying the equipment arranged along therecycle line, which comprises only a compressor and a beat exchanger,and avoiding the use of injection means of condensed liquid directlyprotruding into the fluidized bed.

The process according to the present invention is particularly suitablefor the manufacture of polymers or copolymers of α-olefins, such as highdensity polyethylene (HDPE), linear low density polyethylene (LLDPE),polypropylene (PP), random copolymers (RACO) of ethylene and propylene,and of ethylene or propylene with other α-olefins, ethylene-propylenerubbers (EPR), ethylene-propylene-diene rubbers (EPDM), heterophasiccopolymers (HECO).

The polymerization is generally carried out at a pressure of between 0.5and 6 MPa and at a temperature of between 30 and 130° C. For instance,for LLDPE production the temperature is suitably in the range 80-90° C.and for HDPE the temperature is typically 85-105° C. depending on theactivity of the catalyst system.

The polymerization process herewith described is not restricted to theuse of any particular family of polymerization catalysts. The inventionis useful in any exothermic polymerization reaction employing anycatalyst, whether it is supported or unsupported, and regardless ofwhether it is in pre-polymerized form.

The polymerization reaction may be carried out in the presence of acatalyst system of the Ziegler-Natta type. Ziegler-Natta catalystsystems are solid catalyst systems comprising the reaction product of:

-   -   A) a solid component comprising a titanium compound supported on        a magnesium halide in active form and optionally an electron        donor compound (inside donor);    -   B) an alkyl aluminum compound, optionally in the presence of an        electron donor compound (outside donor).

Suitable titanium compounds are Ti halides (such as TiCl₄, TiCl₃), Tialcoholates, Ti haloalcoholates. Such high-activity catalyst systems arecapable of producing large amounts of polymer in a relatively short timeavoiding the step of removing catalyst residues from the polymer.

Other useful catalysts are the vanadium-based catalysts, which comprisethe reaction product of a vanadium compound with an aluminum compound,optionally in the presence of a halogenated organic compound. Optionallythe vanadium compound can be supported on an inorganic carrier, such assilica, alumina, magnesium chloride. Suitable vanadium compounds areVCl₄, VCl₃, VOCl₃, vanadium acetyl acetonate.

Other suitable catalysts are single site catalysts, i.e. compounds of ametal belonging to groups IIIA to VIIIA (IUPAC notation) of the PeriodicTable of the Elements, including elements belonging to the group of therare earth, linked with a π bond to one or more cyclopentadienyl typerings, utilized with a suitable activating compound, generally analumoxane, such as those described in EP 129 368. As an example ofsingle site catalysts, the “constrained geometry” catalysts can be used,such as those disclosed in EP 416 815. Well-known constrained geometrycatalysts are described in EP-A-0 416 815, EP-A-0 420 436, EP-A-0 671404, EP-A-0 643 066 and WO-A-91/04257. Also Metallocene complexes can becited as single-site catalysts, such as those described in WO 98/22486,WO 99/58539 WO 99/24446, U.S. Pat. No. 5,556,928, WO 96/22995,EP-485822, EP-485820, U.S. Pat. No. 5,324,800 and EP-A-0 129 368.Heterocyclic metallocenes, such as those described in WO 98/22486 and WO99/24446, can be also used.

Other useful catalysts are those based on chromium compounds, such aschromium oxide on silica, also known as Phillips catalysts.

The catalyst may suitably be employed in the form of a pre-polymerpowder prepared beforehand during a pre-polymerization stage with theaid of a catalyst as described above.

The pre-polymerization may be carried out by any suitable process, forexample, polymerization in a liquid hydrocarbon diluent or in the gasphase using a batch process, a semi-continuos process or a continuesprocess.

1-14. (canceled)
 15. A continuous process for polymerizing olefins in afluidized bed reactor, said process comprising: continuously passing agaseous stream comprising one or more α-olefin monomers through afluidized bed with a polymerization catalyst under reactive conditions;withdrawing a polymeric product and unreacted fluids from said fluidizedbed reactor; cooling part of said unreacted fluids below at least onedew point of said unreacted fluids to form a two-phase mixturecomprising a gas and a condensed liquid; and reintroducing saidtwo-phase mixture into said fluidized bed reactor, wherein saidtwo-phase mixture is reintroduced under a distribution plate of saidfluidized bed reactor such that at least one part of said condensedliquid is separated from said gas, and said condensed liquid is fedabove said fluidized bed through an external pipe connecting a bottom ofsaid fluidized bed reactor to a position above an upper limit of saidfluidized bed.
 16. The process according to claim 15, wherein saidtwo-phase mixture is reintroduced under said distribution plate in adirection which is tangential to at least one reactor wall.
 17. Theprocess according to claim 15, wherein at least one part of saidcondensed liquid is separated from said gas by a centrifugal effectforming a separated liquid.
 18. The process according to claim 15,wherein at least one part of said condensed liquid is separated fromsaid gas by a coalescence of liquid droplets and a consequent fall bygravity forming a separated liquid.
 19. The process according to claim15, wherein a separated liquid collects at a zone underlying saiddistribution plate before entering said external pipe.
 20. The processaccording to claim 15, wherein a separated liquid enters said externalpipe, said separated liquid comprises from 20 to 50% by weight of saidcondensed liquid.
 21. The process according to claim 20, wherein saidseparated liquid flows upward in said external pipe without requiring apumping device.
 22. The process according to claim 20, wherein saidseparated liquid is introduced into said fluidized bed reactor at aposition situated above said upper limit of said fluidized bed and belowa velocity reduction zone.
 23. The process according to claim 20,wherein said separated liquid is sprayed into said fluidized bed by aninjection device.
 24. The process according to claim 20, wherein saidseparated liquid comprises from 10 to 20% by weight of said gas enteringsaid external pipe.
 25. The process according to claim 15, wherein saidexternal pipe has a diameter from 0.01 D_(R) to 0.15 D_(R), whereinD_(R) is said fluidized bed reactor diameter.
 26. The process accordingto claim 15, wherein at least one part of said condensed liquid enterssaid fluidized bed by passing through said distribution plate.
 27. Theprocess according to claim 15, wherein a gaseous stream is continuouslypassed through said fluidized bed, said gaseous stream comprises one ormore monomers of formula CH₂═CHR, wherein R is hydrogen or a hydrocarbonradical having 1-12 carbon atoms.
 28. The process according to claim 27,wherein said gaseous stream includes at least one C4-C8 alkane orcycloalkane as an inert condensable gas.